An azeotropic phenomenon takes place in a combination of two or more components which may provide a maximum azeotropic point or a minimum azeotropic point. In an “azeotropic distillation” utilizing the azeotropic phenomenon, when a substance (generally called an “entrainer” or “auxiliary agent”) capable of forming an azeotropic mixture with at least one of any of at least two substances, which are difficult to be separated from each other by simple distillation or fractional distillation, is added to the mixture comprising the above-mentioned at least two substances, the separation performance of the distillation may be improved. Examples of the industrial fields in which the azeotropic distillation is applicable, may include: a step for producing allyl alcohols through the hydrolysis reaction of allyl acetate (Patent Document 1 and Patent Document 2), or a step for purifying 2,3-dichloro-1-propanol (Patent Document 3).
Patent Document 1 discloses a method for collecting allyl acetate from an azeotropic mixture comprising allyl acetate (which is one reactant component for a hydrolysis reaction), water, and allyl alcohol (which is a reaction product), through an extraction operation. However, in this Patent Document 1, all of the amount of the collected allyl acetate is supplied to the reaction step, and Patent Document 1 does not teach or suggest an effect of supplying the allyl acetate as an entrainer to a distillation column.
Patent Document 2 discloses an azeotropic distillation method, wherein allyl acetate, which is one reactant component of a hydrolysis reaction, is used as an azeotropic entrainer for water separation. However, in the same manner as in Patent Document 1, Patent Document 2 does not teach or suggest an effect of supplying the allyl acetate as an entrainer to a distillation column.
On the other hand, Patent Document 3 discloses an azeotropic distillation method, wherein a by-product of the reaction is concentrated and the resultant concentrate is used as an entrainer. However, in this Patent Document 3, the entrainer used is not a reactant, and therefore the entrainer is not recycled to the reaction step.
Hereinbelow, each the Patent Documents is described in more detail.
FIG. 1 is a flow diagram for illustrating the process of Patent Document 1. Referring to FIG. 1, in such a process, a reactant (103) which comprises a reactant (101) mainly comprising allyl acetate and water, and a collected (or recycled) allyl acetate (102) which has been mixed with the reactant (101), is introduced into a hydrolysis reactor (11). A reaction product liquid (104) which has been taken out from the hydrolysis reactor (11) is introduced into the first distillation column (12), and is subjected to distillation therein, and the bottom liquid (105) including aqueous acetic acid, etc., is discharged from the distillation column, and the discharged liquid is recycled into the allyl acetate producing step.
On the other hand, a distilled azeotropic mixture (106) comprising allyl alcohol, allyl acetate and water is discharged from the top of the first distillation column (12), and is introduced into a decanter (13). In the decanter, the above-mentioned mixture (106) is separated into two layers: an oil layer (110) which is rich in allyl acetate, and an aqueous layer (109) which is poor in allyl acetate. The oil layer (110) is introduced to an extraction tower (14). In this extraction tower (14), the allyl acetate is removed by means of an extraction operation using an extraction agent (112) mainly comprising water. The bottom liquid (111) mainly comprising allyl alcohol and water, and the bottom liquid is refined to allyl alcohol including a slight amount of water in a post-process distillation operation. On the other hand, a liquid (113) comprising water and allyl acetate containing substantially no allyl alcohol is taken out from the top of the extraction column (14), and is supplied to the reactor (11) as the above-mentioned recycled liquid (102).
FIG. 2 is a flow diagram for illustrating the process of Patent Document 2. Referring to FIG. 2, a reactant liquid (201) mainly comprising allyl acetate and water, a recycled liquid (202) comprising allyl acetate as a recycled reactant component and water are passed, in combination, through a hydrolysis reactor (21) so as to cause a hydrolysis reaction of allyl acetate, to thereby obtain allyl alcohol and acetic acid. The reaction product liquid (204) produced by the reactor (21) comprises allyl acetate, allyl alcohol, water and acetic acid, and is supplied to the first distillation column (22). A portion of unreacted allyl acetate is collected from the top of the first distillation column (22) as an azeotropic mixture of allyl acetate, water and allyl alcohol, and a distillate liquid (208) is recycled to the above-mentioned hydrolysis reactor (21).
On the other hand, the thus produced acetic acid, allyl alcohol, the unreacted water, and a portion of the unreacted allyl acetate are taken out from the bottom of the first distillation column (22). The liquid (205) taken out from the bottom of the first distillation column (22) is supplied to the second distillation column (23). By use of the allyl acetate in the liquid (205) as an azeotropic entrainer, an azeotropic mixture liquid (212) of allyl alcohol, water and allyl acetate is obtained from the top of the column, and a liquid (209) mainly comprising acetic acid, or acetic acid and water is obtained from the bottom of the column. The fraction of distillate (212) at the column top of the second distillation column (23) is supplied to the third distillation column (24).
Water, allyl acetate and allyl alcohol are discharged from the top of the third distillation column (24), and after the condensation of such a mixture, the mixture is separated into two layers (namely, an oil layer and an aqueous layer) in a decanter (25). The oil layer mainly comprises allyl acetate. A portion or all of this organic phase is recycled to the column (24) as an azeotropic entrainer so as to purify the allyl alcohol. The aqueous layer contains small amounts of allyl alcohol and allyl acetate and is returned to the hydrolysis reactor (21) together with a portion of the above-mentioned oil layer.
FIG. 3 is a flow diagram for illustrating the process of Patent Document 3. Referring to FIG. 3, a liquid (301) in this process mainly comprising an intermediate product (namely, 2,3-dichloro-1-propanol (hereinafter, referred to as “DCH”)) to be produced in an epichlorohydrin-producing process using allyl alcohol as a reactant, and also contains 1,2,3-trichloropropane (hereinafter, referred to as “TCP”) as a by product, and other low-boiling point substances. The liquid (301) is supplied to a first distillation column (31), and a liquid (307) mainly comprising TCP is supplied from the top of the first distillation column (31) as an entrainer. Because of the presence of the TCP supplied from the top of the column which acts as an entrainer, the water in the liquid (301) is formed into a TCP-water azeotropic mixture having a boiling point which is lower than that of a DCH-water azeotropic mixture, is subjected to distillation and moved toward the top of the column, so as to provide a column-top distillate (303) from the first distillation column (31). Consequently, a large portion of the water is formed into the TCP-water azeotropic mixture in the distillation, and therefore the production of distilled DCH at the top of the column is suppressed. The liquid (302) at the bottom of the column is supplied to the refining equipment to be used in a subsequent process. The column-top distillate (303) is condensed, cooled and is separated into an aqueous layer (304) and an oil layer (305) in a decanter (32). The aqueous layer (304) is supplied to a separately provided processing equipment. The oil layer (305) is supplied to the second distillation column (33), and supplied from the bottom of the column to the first distillation column (31) as a liquid (307) mainly comprising TCP. The column-top distillate (306) is introduced to a separately provided processing equipment.
In recent years, in view the need for reduction in carbon dioxide emissions and for fuel saving, there are intense demands for the reduction in the energy to be required for the separation in azeotropic distillation without substantively decreasing the separation performance therein.
[Patent Document 1] JP-A (Japanese Unexamined Patent Publication; Kokai) No. 62-149637
[Patent Document 2] JP-A No. 1-85940
[Patent Document 3] JP-A No. 7-25796